1. Field of the Invention
The present invention relates to a liquid crystal device and, more particularly, to a ferroelectric liquid crystal device having both a high-speed response characteristic and a memory effect and a method of driving the same.
2. Description of the Prior Art
It is expected that a ferroelectric liquid crystal is applied to a high-speed liquid crystal shutter for a printer and large-capacity display by utilizing its memory effect and high-speed response characteristic.
Most of ferroelectric liquid crystal devices manufactured as test samples so far are those in which a ferroelectric liquid crystal is sandwiched between two glass plates each having alignment films composed of rubbed organic polymer films or oblique deposited inorganic insulating member as of SiO on opposing surfaces. For example, Japanese Patent Laid-Open No. 62-173433 discloses a liquid crystal device using a rubbed polyimide thin film. In a liquid crystal device of this type, all the liquid crystal molecules are aligned in the same direction to be parallel to the glass substrate surfaces, i.e., in a uniform aligned state upon application of a voltage. When a driving voltage is reset to zero, the uniform alignment is disordered, and the liquid crystal molecules define a twisted structure between the glass substrates, i.e., are in a twisted aligned state. As a result, a memory effect is considerably impaired.
In contrast to this, as in a method disclosed in Japanese Patent Laid-Open No. 62-250418, a liquid crystal device in which two substrates having alignment films formed by SiO oblique deposition are arranged so that each direction of deposition in the same inclination state can provide a memory effect. However, memory nonuniformity locally occurs, and uniform switching characteristics are difficult to obtain.
Japanese Patent Laid-Open No. 62-160420 discloses a liquid crystal device in which stable states of liquid crystal molecules include two different uniform aligned states and a twisted aligned state. It is demonstrated that this liquid crystal device does not much suffer from display nonuniformity caused by memory nonuniformity. However, a definite structure of a liquid crystal device which can take three stable display states and a satisfactory method of driving the same have not been developed yet.
Another problem posed in a practical application of a ferroelectric liquid crystal device is that a bright state of a ferroelectric liquid crystal device is undesirably colored in a variety of colors depending on refractive index anisotropy of liquid crystal molecules and a thickness of a liquid crystal layer, and the ferroelectric liquid crystal device is not easy to perform black-and-white display.
Furthermore, when a ferroelectric liquid crystal device is put into a practical application, continuous gray scale control of a transmittance in a multiplex driving mode poses a serious technical problem. For example, Japanese Patent Laid-Open No. 62-160420 tries gray scale display in the multiplex driving mode by utilizing three states, i.e., two different uniform aligned states and a twisted aligned state. However, in this prior art, only three gray scale levels can be displayed. Japanese Patent Laid-Open No. 59-193427 discloses a technique in which the number of driving pulses is changed to cause bistable states to coexist, thereby making an attempt of gray scale display. However, according to the experiments of the present inventors, halftone level changed over time in the multiplex driving mode, and stable halftone display could not be obtained.